It is impossible to determine the condition of the coolant in the radiator just by looking at it. Coolant, a mixture of ethylene glycol and water, breaks down with age, picks up contaminants that cause sludge, and becomes acidic. When this happens, it can cause corrosion within the radiator and cooling passages of the engine. To determine its condition, coolant must be checked with coolant test strips that measure PH balance. Coolant is an environmentally hazardous substance. It pollutes the water table and is poisonous to people and animals and therefore must be disposed of as a hazardous waste. Your mechanic has special tools and procedures for testing and changing coolant.